The present invention relates to a method for operating a laser system in a Q-switched mode, thereby generating a train of at least two subsequent laser pulses, wherein said laser system has a laser resonator with a laser medium and an electro-optical modulator, wherein the electro-optical modulator comprises an EOM crystal, wherein the EOM crystal has, when subjected to acoustic ringing, a characteristic ringing time, and wherein the EOM crystal is driven by modulator voltage pulses having a modulator voltage pulse duration,
For numerous applications laser systems are required to generate a train of subsequent laser pulses. The laser system comprises a laser medium, e.g. a laser crystal, which is pumped by a flashlamp in order to generate such laser pulses. In the common free running operation mode the laser pulses follow the flashlamp pulses with some time delay, thereby approximately resembling the flashlamp pulses to some extent in form and frequency. However, the free running operation mode is limited with respect to high peak powers and very short pulse durations.
In order to achieve high peak powers and/or shorter pulse durations, the laser systems further comprises an electro-optical modulator (EOM), e.g. a Pockels cell, which enables a laser resonator to operate in a quality switched (Q-switched) mode. In combination with a polarizer and optionally a quarter wave plate it serves as an optical switch to enable generation of short and high power laser pulses having peak powers some orders of magnitude higher compared to a common free running mode of operation.
The electro-optical modulator (EOM) typically comprises a Pockels cell. The basic element of a Pockets cell is an optical crystal (EOM crystal as referred to herein), which exhibits birefringence when an electric field is applied. This influences polarization of a laser beam in a resonator and by applying a well defined voltage pulse the optical switch can be driven from a closed to an open state. If transition from a closed to an open state is made fast and energy is already accumulated in the laser medium, this results in an intense build-up of a Q-switched laser pulse.
Application of the modulator voltage pulse on the EOM crystal, however, not only changes the optical properties of the crystal, but also causes the crystal to change its mechanical properties due to the piezoelectric effect. When voltage is applied quickly (in a step function) the EOM crystal starts to oscillate, changing its longitudinal dimensions. This phenomenon is usually referred to as acoustic ringing and results in oscillations of the optical switch properties around the stationary value with characteristic ringing time or ringing period, which dies out with a characteristic damping time. The characteristic damping time is usually orders of magnitude higher compared to the characteristic ringing time.
During the ringing period the switching properties of the optical switch (EOM) are not stable and no proper control of the laser cavity losses is possible. If a second Q-switched pulse is to be generated when the ringing effect is still present, this influences the second pulse stability to a great extent. Additionally, during the pumping sequence of the second pulse pre-lasing action can occur because of the oscillating behaviour of the Q-switch. This effect can therefore limit the operation of Q-switched lasers when the repetition rates, i.e. modulator pulse periods shorter than the characteristic damping time are desired.
Different crystals can be used in the EOM exhibiting more or less pronounced piezoelectric effect (KD*P, BBO, lithium niobate, . . . ). Generally, more pronounced birefringence effect is associated with more prominent piezoelectric effect. In consequence the achievable repetition rate of Q-switched laser pulses is significantly limited by said piezoelectric effect and the resultant acoustic ringing.
Methods that suppress acoustic ringing and therefore extend the repetition rate limit of the Q-switching have been extensively studied in the past. U.S. Pat. No. 4,221,463 proposes to filter the ringing oscillations by the combination of a second EO crystal and a polarizer. Appropriate voltage is applied to the second crystal at the trailing edge of the laser pulse to filter the ringing out of the laser cavity. According to U.S. Pat. No. 5,221,988 a pair of dielectric panes are attached to opposite sides of the EO crystal. The panes are in contact with aluminum blocks, which mechanically dampen the ringing oscillations. Another way of mechanically reducing the ringing oscillation effect is described in U.S. Pat. No. 7,324,266.
The invention has the object to avoid ringing oscillations of the EOM crystal in a Q-switched mode operation of a laser system when generating a train of at least two subsequent laser pulses.